The present invention relates to a master cylinder means to be incorporated in the oil hydraulic braking system of an automobile.
Conventionally, the oil hydraulic braking system of an automobile incorporates a master cylinder means of a linear cylinder-piston type having a cylinder tube and a piston, wherein the piston is axially reciprocated in a cylinder chamber defined in the cylinder tube. When the brake pedal is stepped on, the piston is linearly shifted relative to the cylinder tube, whereby brake fluid (oil) is delivered under pressure toward brake cylinders incorporated in the braking mechanisms of individual wheels. In this conventional linear type master cylinder means, the rate of delivery of fluid relative to the amount of stepping-on of the brake pedal is substantially constant as shown by the solid line in FIG. 1. In this case, due to the incompressible character of the brake fluid (oil), the pressure generated in the braking system relative to the amount of stepping-on of the brake pedal shows a performance such as shown by the solid line in FIG. 2, wherein the pressure remains at a relatively low level when the brake pedal is stepped on starting from its completely released position until it cancels the clearance between the brake shoes and the brake drums, and when the brake shoes have all contacted the brake drums, the oil pressure begins to increase abruptly when the brake pedal is further stepped on for a relatively small amount.
Therefore, in the conventional linear type master cylinder means, the stepping-on stroke of the brake pedal is mostly consumed without imparting any substantial braking effect to the vehicle, and on the other hand in the final stage of the stepping-on stroke of the brake pedal an abruptly increasing braking action is imparted to the vehicle. Because of this braking performance, the brake pedal requires delicate operation which is very difficult for drivers, particularly beginners, to learn.
In order to obtain a desirably progressive braking effect by easy operation of the brake pedal, the oil pressure generation in the braking system relative to the amount of stepping-on of the brake pedal should have a performance such as shown by the broken line in FIG. 2. In order to obtain this performance, it is required that the rate of delivery of brake fluid relative to the amount of stepping-on of the brake pedal should have a performance such as shown by the broken line in FIG. 1. This means that in the initial stage of the stepping-on of the brake pedal, the brake fluid is delivered at a relatively high rate and the rate of delivery gradually lowers as the brake pedal is further stepped on.